1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device, which compensates for brightness phenomenon in a last row (or column) data line portion.
2. Description of the Related Art
In general, liquid crystal display (LCD) devices are used as portable display devices due to their lightweight, thin, small, and compact characteristics as compared with other display devices. Notebook personal computers are representatives of LCD devices.
These LCD devices include upper and lower glass substrates and a liquid crystal layer interposed therebetween. On the lower glass substrate are disposed a plurality of gate lines arranged along a direction at a constant interval, a plurality of data lines arranged along a direction perpendicular to the gate lines at a constant interval to define pixel regions arranged in a matrix configuration, a plurality of pixel electrodes formed within the respective pixel regions, and a plurality of thin film transistors formed at the respective pixel regions defined by a cross of the gate line and the data line, for applying a data signal of the data line to the corresponding pixel electrode depending on a signal of the gate lines. In addition, formed on the upper glass substrate are a black matrix layer for preventing incident light from passing through portions other than the pixel regions, a color filter layer for displaying colors in the pixel regions, and a common electrode layer formed on the entire surface of the upper glass substrate including the color filter layer. Interposed between the upper and lower glass substrates is a liquid crystal material layer.
The liquid crystal material layer formed between the upper and lower glass substrates may deteriorate if a DC voltage is applied for a long time. To prevent this deterioration, the polarity of the applied voltage is periodically changed during operation, which is called “polarity inversion driving method.” The polarity inversion driving method includes frame inversion, line inversion, column inversion, dot inversion and so forth.
The frame inversion driving method applies a data voltage to the liquid crystal layer such that the polarity of the data voltage with respect to the common electrode voltage is identical in a frame unit. For example, if a data voltage having a positive (+) polarity is applied to even frames, a data voltage having a negative (−) polarity is applied to odd frames. The frame inversion driving method has the advantage that power consumption occurring in the switching operation is small. However, the frame inversion driving method is sensitive to a flicker phenomenon due to asymmetry in the transmittances of the positive and negative polarity. In addition, the frame inversion driving method is susceptible to crosstalk due to interference between data.
The line inversion driving method, which is a type of polarity inversion driving method, is commonly used in low resolution (VGA, SVGA, etc.) display devices, and applies data voltage such that the polarity of the pixel is different in a horizontal line unit. For example, if a data voltage having positive (+) polarity was applied to an odd line, and a data voltage having negative (−) polarity was applied to an even line, a data voltage having negative polarity is applied to an odd line and a data voltage having positive polarity is applied to an even line in the next frame. In the aforementioned line inversion driving method, data voltages having the opposite polarities are applied between adjacent lines, so that a brightness difference between lines decreases due to spatial averaging. Accordingly, the flicker phenomenon is reduced as compared to the frame inversion driving method. In addition, along the vertical direction, data voltages having opposite polarities are distributed, so that a coupling phenomenon between data lines is countervailed. Accordingly, vertical crosstalk is reduced as compared to the frame inversion driving method. However, along the horizontal direction, data voltages having the same polarities are distributed, so that horizontal crosstalk is generated and, since repeating times of the switching increases compared with the frame inversion driving method, power consumption increases.
The column inversion driving method applies data voltage in which polarity is the same along the horizontal direction, but opposite along the vertical direction. Like the line inversion driving method, the column inversion driving method has a smaller flicker phenomenon and a smaller horizontal crosstalk as compared to the frame inversion driving method due to spatial averaging. However, since the column inversion driving method has to apply data voltages having opposite polarities between vertically adjacent lines, a high voltage column drive IC is required.
The dot inversion driving method, which is a type of polarity inversion driving method, is applied to high resolution (XGA, SXGA, UXGA) display devices. In the dot inversion driving method, the data voltages of adjacent pixels along omnibus directions have opposite polarities. Accordingly, the dot inversion driving method minimizes the flickering phenomenon by using spatial averaging, but has a disadvantage in that a high voltage drive IC has to be used and power consumption is very high.
In FIG. 1 is a layout diagram of a liquid crystal display device according to the related art. FIG. 1 a LCD device includes a plurality of gate lines 1 arranged along a first direction and spaced apart at a constant interval, and a plurality of data lines 2 arranged along a second direction perpendicular to the gate lines at a constant interval, thereby forming pixel regions arranged in a matrix configuration. A plurality of pixel electrodes 3 are formed within the respective pixel regions, and a plurality of thin film transistors 4, for applying a data signal of the data line to the corresponding pixel electrode, are formed at the respective pixel regions defined by a cross of the gate line and the data line. The pixel electrodes are formed within the pixel regions defined by gate lines 1 and data lines 2. In addition, most pixel electrodes 3 have data lines 2 at both sides thereof, but the pixel electrodes located in the last row (i.e., right end) have a data line 2 only at one side. Thus, the pixel regions located at the right end have a different brightness and coupling influence than the pixel regions located elsewhere.
FIG. 2 is an exemplary equivalent circuit diagram of a subpixel of the last data line in a liquid crystal display device according to related art. In FIG. 2, subpixels of the last data line of FIG. 1 in the pixel region immediately before the right end data line, a capacitor (CLC) is formed between the pixel electrode and the common electrode, a storage capacitor (Cst) is formed between the prior gate line 1 and the pixel electrode 3, a parasitic capacitor (Cdpl) is formed between the left data line 2 and the pixel electrode 3, and a parasitic capacitor (Cdpr) is formed between the right end data line 2 and the pixel electrode 3. However, in the pixel region of the right end data line, a parasitic capacitor (Cdpr) is not formed between the right end data line 2 and the pixel electrode 3. Accordingly, the pixel regions of the right end data line have different capacitances than the remaining pixel regions, causing the pixel regions of the right end data line to be displayed brighter than the remaining pixel regions.